This invention relates to valve structures which control other mechanisms in response to fluid pressure signals and more particularly to a simplified valve construction which may activate and inactivate a torque converter in the drive line of a vehicle in response to fluid pressure signals indicative of vehicle speed and of transmission gear setting.
Prior U.S. Pat. No. 3,857,302 of Hugh C. Morris, issued Dec. 31, 1974 for Transmission and Speed Controlled Lockup Clutch, relates to a vehicle drive system including a torque converter and a multi-speed transmission and includes a control valve which automatically engages or disengages a lock-up clutch in the torque converter in response to fluid pressure signals indicative of vehicle speed and of transmission gear setting. The control valve couples the torque converter into the drive path under conditions where that is desirable and bypasses the torque converter, to establish solid mechanical drive under other conditions.
More specifically, it is generally desirable that the torque converter be active in the drive path, without regard to vehicle speed, at the lower gear settings of the transmission. At the higher settings of the transmission the torque converter should be active only at low vehicle speed and should be effectively bypassed as vehicle speed increases. As is understood by those skilled in the art, utilization of a torque converter in the vehicle drive line is desirable at start-up and at low vehicle speeds and while operating in one or more of the lowest gear settings of the transmission in that power throughput is maintained fairly constant over a broad range of vehicle speeds. However, this advantage becomes less significant as vehicle speed increases to higher absolute levels at higher gear settings since speed variations are then proportionately less. Solid mechanical drive, realized by bypassing the torque converter, is basically more efficient from the standpoint of power losses, and at the higher speeds and gear settings, this advantage outweighs the broad torque capabilities of fluid drive. Accordingly, the control valve of prior U.S. Pat. No. 3,897,302 automatically achieves optimum driveline performance by pressurizing a torque converter lock-up or bypass clutch as higher vehicle speeds are reached, but does so only when the transmission is at one of the higher gear settings. The control valve performs a further highly useful function by temporarily re-establishing the fluid drive whenever the transmission is being shifted. This reduces shifting shocks by temporarily removing the solid mechanical drive connection between the driving engine and the transmission during the shifting periods. The temporary substitution of a fluid connection serves to cushion such shocks.
It would be very undesirable if the control valve disconnected the torque converter from the drive path as a specific vehicle speed is reached, and then reconnected the converter when vehicle speed decreased again to the same level. Under that condition, if vehicle speed were to oscillate about that particular level, repeated connection and disconnection of the torque converter would occur. This would produce repeated drive line shocks and greatly aggravate wear of components. Accordingly, the control valve of prior U.S. Pat. No. 3,857,302 avoids such chattering or hunting by exhibiting a hysteresis or snap-action effect. In particular, the vehicle speed value at which the torque converter is effectively bypassed is considerably higher than the speed level at which the torque converter is reconnected back into the drive line. Thus, minor oscillations of speed about some one value do not cause repetitive engagements and disengagements of the torque converter lock-up clutch.
In order to realize the functions described above, the control valve of prior U.S. Pat. No. 3,857,302 has a somewhat complex and costly construction. A valve spool assembly having six separable parts is disposed in a bore which must be machined to include seven axially spaced internal grooves. Further, the degree of hysteresis exhibited by the prior valve is somewhat limited and the pressure values at which the snap action is triggered may fluctuate to a limited extent in practice.